Rose Under Fire, by Elizabeth Wein
Last year I read Code Name Verity, and it was amazing. Rose Under Fire is the companion novel; you don't have to have read Verity, but Maggie is in both.
Rose is an American girl and she knows how to fly planes, so she has a job ferrying planes around for repair or transport or whatever. She is nowhere near the front lines, and although she would love to get to Europe, the chances are slim. It's mid-1944 and the Nazis are being beaten back, though, so when France is liberated, Rose gets a chance to ferry a plane from there. Then she disappears.
Rose has been captured by the Germans, who assumed they had a flying spy, not a girl gofer. But they're not about to let her go, so they send her to Ravensbrück, the women's concentration camp. There, she tries to survive, trading bread and getting to know many different people, including the Rabbits--a group of women subjected to horrifying medical experiments.
Just like Code Name Verity, this is a fantastic book. The writing is great, the characters live, and the emotions are real. I hope Wein will be writing books for a long time. Check out her website for some great resources on the Rabbits and other events featured in the book.